1. Field of the Invention
The present invention relates to a method for producing electroluminescent materials used for fabricating GaP light emitting devices, which materials have a structure of a plurality of GaP layers epitaxially grown on a GaP single crystal substrate (hereafter referred to as GaP epitaxial wafer).
2. Description of the Prior Art
In general, GaP light emitting diodes are fabricated in such a manner that first one or more GaP crystal layers of both n-type and p-type are consecutively one by one laminated on an n-type GaP single crystal substrate to produce a GaP epitaxial wafer including a pn junction and then said GaP epitaxial wafer is processed into the devices. In the mean time, GaP light emitting diodes are largely divided into three kinds in regard to the colors of emitted light, such as a green GaP light emitting diode (or yellowish green GaP light emitting diode), a red GaP light emitting diode and a pure green GaP light emitting diode.
A green GaP light emitting diode 2 is fabricated from a GaP epitaxial wafer as shown in FIG. 2, in which Nitrogen atoms (N) serving as luminescence centers are doped in the portion of the n-type GaP layer in the vicinity of the pn junction and emits yellowish green light with the peak wavelength of about 567 nm.
A red GaP light emitting diode 4 is fabricated from a GaP epitaxial wafer as shown in FIG. 3, in which Zinc (Zn) and Oxygen (O) are both doped in the p type layer to form Zn-O pairs serving as luminescence centers and emits red light with the peak wavelength of about 700 nm.
A pure green GaP light emitting diode 6 is fabricated from a GaP epitaxial wafer as shown in FIG. 4, which has no dopant as luminescence centers therein and emits pure green light with the peak wavelength of about 555 nm.
The GaP epitaxial wafer 2 used for fabricating green light emitting diodes, the GaP epitaxial wafer 4 used for fabricating red light emitting diodes and the GaP epitaxial wafer 6 used for fabricating pure green light emitting diodes are illustrated in sectional structure respectively in FIGS. 2, 3 and 4, as an example for each. The GaP epitaxial wafer 2 for green light diodes has a structure in which an n-type GaP buffer layer 11a, an n-type GaP layer 12a, a Nitrogen doped n-type GaP layer 13a and a p-type GaP layer 14a are consecutively in that order formed on an n-type GaP single crystal substrate 10a; the GaP epitaxial wafer 4 for red light emitting diodes has a structure in which an n-type GaP layer 11b and a p-type GaP layer 14b including Zn-O pairs therein are consecutively formed on an n-type GaP single crystal substrate 10b; and the GaP epitaxial wafer 6 for pure green light emitting diodes has a structure in which an n-type GaP buffer layer 11c, an n-type GaP layer 12c, an n-type GaP layer or a p-type GaP layer with a lower carrier density and a p-type GaP layer 14c are consecutively in that order formed on an n-type GaP single crystal substrate 10c.
Among methods by which GaP layers are consecutively formed on a GaP single crystal substrate or a GaP layer(s) which has been formed on the GaP single crystal substrate in the preceding step(s) as mentioned above, an established method is a liquid phase epitaxial method by cooling a saturated Ga solution of GaP (hereinafter referred to as liquid phase epitaxial method or liquid phase epitaxial growth). The liquid phase epitaxial method is further divided into two methods, which consist of a melt-back method and a non-melt-back method. The non-melt-back method is performed in the following way: GaP polycrystal is dissolved into Ga melt at, for example, 1060.degree. C. to prepare Ga solution saturated with GaP as solute at 1060.degree. C. Then a GaP substrate is contacted with the Ga solution and both of them are gradually cooled so that the GaP solute in the Ga solution is deposited on the GaP substrate as a grown GaP layer. On the other hand the melt-back method is performed in the following way: a GaP substrate is contacted with Ga melt. Then both of them are together heated up to, for instance, 1060.degree. C. so that the upper portion of the GaP substrate is dissolved into the Ga melt to form Ga solution saturated with GaP as solute at 1060.degree. C., and thereafter the substrate and the Ga solution are both gradually cooled to have a GaP layer grown on the GaP substrate in the same manner as in the non-melt-back method.
A GaP epitaxial wafer for light emitting diodes is usually produced by way of a liquid phase epitaxial method consisting of two steps (hereinafter specially referred to as two step method).
Referring to FIG. 2, the two step method will be described first taking as an example a GaP epitaxial wafer 2 used for fabricating green light emitting diodes. In the first step, an n-type GaP buffer layer 11a is formed on an n-type GaP single crystal substrate 10a by a non-melt-back method or a melt-back method (hereinafter the thus processed substrate is referred to as a layered substrate). In the second step, the melt-back method is applied to the layered GaP substrate. That is, an n-type GaP layer 12a, a Nitrogen doped n-type GaP layer 13a and a p-type GaP layer 14a are further consecutively in that order formed on the n-type GaP buffer layer 11a of the layered GaP substrate the melt-back method.
In the cases of a two step method GaP epitaxial wafer for red light emitting diodes and a two step method GaP epitaxial wafer for pure green light emitting diodes, layered GaP substrates are an n-type GaP layer 11b on an n-type GaP single crystal substrate 10b and an n-type GaP buffer layer 11c on an n-type GaP single crystal substrate 10c respectively.
Hereinafter a plurality of GaP layers formed by a liquid phase epitaxial growth on a layered GaP substrate is generically named a GaP light emitting layer composite. For example, in case of a GaP epitaxial wafer 2 for green light emitting diodes as shown in FIG. 2, three layers of an n-type GaP layer 12a, a Nitrogen doped n-type GaP layer 13a and a p-type GaP layer 14a are called a GaP light emitting layer composite 19a of the GaP epitaxial wafer 2 for green light emitting diodes, while in case of a GaP epitaxial wafer 6 for the pure green light emitting diodes as shown in FIG. 4, three layers of an n-type GaP layer 12c, an n-type GaP layer or a p-type GaP layer 13c with a lower carrier concentration and a p-type GaP layer 14c are called a GaP light emitting layer composite 19c of the GaP epitaxial wafer 6 for pure green light emitting diodes. A p-type GaP layer 14b, which is of a single layer, of a GaP epitaxial wafer 4 for red light emitting diodes as shown in FIG. 3 is also called a GaP light emitting layer composite 14b of the GaP epitaxial wafer 4 for red light emitting diodes.
In the first step of the two step method, more than one GaP layers may be formed on a GaP single crystal substrate in order to further improve a characteristic(s) of a light emitting diode, though a monolayered GaP substrate is taken up as an example for the purpose of illustration only in the description above.
A two step method with a layered GaP substrate having more than one layers therein is conventionally considered useful for achieving higher brightness. However, even a thus produced GaP epitaxial wafer for light emitting diodes has still a problem by which said GaP epitaxial wafer can not achieve desired brightness of the light emitting diodes therefrom under the present requirement for increasingly higher brightness.
As the results of the research conducted by the inventors in regard to the factors lowering brightness of a light emitting diode, the following discoveries have been obtained that surface irregularities accompanying crystallographic defects are generated on the surface of a GaP layer formed on a GaP single crystal substrate and the surface irregularities give an ill influence to deteriorate the crystallinity of a following light emitting layer composite, which is formed on said layered GaP substrate in the following step of liquid phase epitaxial growth. Deterioration of said composite necessarily results in lowering the brightness of light emitting diodes.